The Specific Aim of the proposed research is to better understand the relationship between physical growth and the mass-related gross aerobic demand (VO2) of locomotion in pre-pubescent children. The investigators plan to employ a longitudinal design, featuring yearly data collection on 30 to 44 children over a 5-year period (beginning at age 6 and ending at age 10). Each yearly data collection effort will consist of six testing sessions. In Sessions 1 and 2, subjects will accommodate to level treadmill walking and running at 1.56 m.s-1 and 2.23 m.s-1, respectively. In Session 3, body surface area to body mass ratio, resting VO2 exercise ventilatory equivalent, and the metabolic cost of level treadmill walking and running will be measured.Gait descriptors linked to economical locomotion will also be quantified during treadmill and overground walking and running trials. In Session 4, the VO2 of level treadmill walking will be measured at the freely-chosen step length (FCSL) and at shorter and longer SLs relative to the FCSL, while the last two sessions will feature assessment of segmental inertial characteristics (Session 5) and skeletal age (Session 6). Data gathered each year will be analyzed using correlational and multiple regression procedures to test the following hypotheses: as young children grow physically, the mass-related decrease in gross aerobic demand required to walk or run a given distance is associated with a) a reduction in exercise ventilatory equivalent (Hypothesis #1); b) a reduction in body surface area to body mass ratio (Hypothesis #2); c) a smaller moment of inertia of the leg about a transverse axis through the hip joint during the swing phase and smaller proportion of total body mass concentrated in the legs (Hypothesis #3); and/or d) an increase in ground-contact time and step length and a decrease in step frequency and average relative vertical ground reaction force (Hypothesis #4). It is suggested that analysis of SL/cadence settings and attendant VO2 responses for each age group will also allow the applicants to determine whether children select step length/cadence patterns that minimize the mass-related gross aerobic demand of walking (Hypothesis #5). The use of a multivariate approach to examine maturity-related processes underlying improved economy of movement in young children should yield fundamental knowledge regarding the association between physical growth and the aerobic demand of basic gait patterns. Access to benchmark longitudinal data derived from the proposed investigation may also aid in establishing realistic goals for locomotor energy demands in young children with metabolic (e.g. obesity), structural (e.g. lower extremity amputees), or gait disorders (e.g., cerebral palsy) typified by a high aerobic demand of locomotion and potentially provide an informed basis for the evaluation, treatment, and rehabilitation of these cohorts that accounts for the influence of physical growth.